World

BEIJING — The world’s largest radio telescope began operating in southwestern China on Sunday, a project Beijing says will help humanity search for alien life. 

The Five-hundred-metre Aperture Spherical Radio Telescope (FAST), nestled between hills in the mountainous region of Guizhou, began working around noon, the official Xinhua news agency reported. 

Built at a cost of 1.2 billion yuan ($180 million), the telescope dwarfs the Arecibo Observatory in Puerto Rico as the world’s largest single-dish radio telescope, with twice the sensitivity and a reflector as large as 30 football fields, it said.

FAST will use its vast dish, made up of 4,450 panels, to search for signs of intelligent life, and to observe distant pulsars — tiny, rapidly spinning neutron stars believed to be the products of supernova explosions. 

China sees its ambitious military-run, multi-billion-dollar space programme as symbolising the country’s progress. It plans a permanent orbiting space station by 2020 and eventually a manned mission to the moon.

Chinese President Xi Jinping celebrated the launch, with reports on Sunday that he had sent a congratulatory letter to the scientists and engineers who contributed to its creation. 

The telescope represents a leap forward for China’s astronomical capabilities and will be one of several “world-class” telescope projects launched in the next decade, said Yan Jun, head of China’s National Astronomical Observation (NAO), according to Xinhua.

In a test run before the launch, FAST detected electromagnetic waves emitted by a pulsar more than 1,300 light-years away, state media reported an NAO researcher as saying. 

Earlier Xinhua cited Wu Xiangping, director-general of the Chinese Astronomical Society, as saying that the telescope’s high degree of sensitivity “will help us to search for intelligent life outside of the galaxy”.

Experts have been hunting for alien intelligence for six decades, pointing radio telescopes at stars in the hope of discovering signals from other civilisations, but have not yet found any evidence.

‘Wildest imagination’ 

Last month a “strong signal” detected by a Russian telescope searching for extraterrestrial signals stirred interest among scientists, but experts said it was far too early to make conclusions about its origin.

But the new FAST telescope could “lead to discoveries beyond our wildest imagination”, Douglas Vakoch, president of METI, a group seeking to send messages to space in search of alien life, told Xinhua. 

Construction of FAST began in 2011, and local officials relocated nearly 10,000 people living within five kilometres to create a quieter environment for monitoring. Cell phones in the area must be powered off to maintain radio silence.

In the past China has relocated hundreds of thousands of people to make way for large infrastructure projects such as dams and canals. 

The area surrounding the telescope is remote and relatively poor. State media said it was chosen because there are no major towns nearby.

The villagers will be compensated with cash or housing. The budget for relocation is 1.8 billion yuan ($270 million), it was reported, more than the cost of constructing the telescope. 

China has poured money into big-ticket science and technology projects as it seeks to become a high-tech leader, but despite some gains the country’s scientific output still lags behind.

At the beginning of this month, reports said 600 apartments had been built so far with the funds.

NEW YORK — The planned US deployment of the THAAD anti-missile system in South Korea is not negotiable as part of efforts to agree new UN sanctions on North Korea after its fifth nuclear test, but Washington is confident tougher steps will be agreed before long, the senior US diplomat for Asia said on Friday.

China, whose full backing is widely seen as crucial for sanctions on North Korea to be effective, is strongly opposed to the deployment of the Terminal High Altitude Area Defence system and some experts have argued it should be part of talks on new UN measures.

Asked whether THAAD was negotiable, Daniel Russel, the US assistant secretary of state for East Asia, referred to a US-South Korean agreement on the deployment.

"No. The two countries have made a decision," he told Reuters.

US Secretary of State John Kerry, meeting Southeast Asian leaders in New York on the sidelines of the UN General Assembly, said the United States would "do whatever is necessary to defend our own citizens and to honour our security commitments to our allies".

 Discussions are under way on a possible new UN sanctions resolution on North Korea after it conducted its fifth and largest nuclear test on September 9.

Russel later told a news briefing discussions were still at an early stage, but he was confident that a new UN resolution would be agreed before long, imposing further sanctions and tightening existing ones.

Among the aims, he said, would be to prevent North Korea's abuse of international infrastructure, including banking and shipping, to further its nuclear programme.

China is North Korea's main ally, but has been angered by its repeated missile and nuclear tests and backed tough UN sanctions on Pyongyang in March.

Questions remain

Beijing has said it will work within the United Nations to formulate a necessary response to the latest nuclear test, but questions remain as to whether it is willing to agree tough enough steps to force North Korea to abandon nuclear weapons.

Two experts from the Massachusetts Institute of Technology's Centre for International Studies last week argued that if China were to agree to serious graduated sanctions on North Korea, the United States could agree to freeze the number of ground-based missile interceptors on the Korean Peninsula.

Eric Heginbotham and Richard Samuels said that as part of a set of incentives to China, Washington "might also agree, after consulting South Korea, to withdraw THAAD from the peninsula when North Korean nuclear weapons no longer pose a threat".

On Monday, Washington said US President Barack Obama and China's Premier Li Keqiang agreed in a meeting on the sidelines of the UN assembly to step up cooperation in the UN Security Council and in law enforcement channels.

China's Foreign Ministry later said a Chinese conglomerate, the Liaoning Hongxiang Group was under investigation following the provisions of the sanctions agreed in March.

In his General Assembly speech on Thursday, South Korea's Foreign Minister Yun Byung-se accused North Korea of "totally ridiculing" the United Nations through its nuclear and missile tests and said it was time to reconsider whether it was qualified for UN membership.

Russel said it was "not unnatural" that such questions should be raised. He stopped short of endorsing the call, but added: "The international system is being exploited by [North Korea]... for the purpose of pursuing an illegal nuclear and missiles programme that threatens both its neighbours and regional peace and security."

UNITED NATIONS, United States — North Korea's foreign minister on Friday told the United Nations that "going nuclear" is his country's only way to defend itself and vowed to further bolster its nuclear military forces. 

Speaking to the General Assembly, Ri Yong-ho said his country will "continue to take measures to strengthen its national nuclear armed forces in both quantity and quality".

 He spoke just two weeks after North Korea's fifth and most powerful nuclear test provoked worldwide condemnation, prompting the UN Security Council to begin work on a new sanctions resolution.

"Going nuclear armed is the policy of our state," Ri, who has been foreign minister since May, told the world gathering.

"As long as there exists a nuclear weapon state in hostile relations with the DPRK [North Korea], our national security and the peace on the Korean peninsula can be defended only with reliable nuclear deterrence," he said.

North Korea has conducted two nuclear tests and test-fired more than 20 missiles this year alone.

After the latest blast on September 9, Pyongyang claimed it had significantly advanced its ability and tested a miniaturised nuclear bomb for a warhead that could be mounted on a missile.

In his address, Ri acknowledged that the nuclear tests "may not be easily understood by European countries", which he said were now "less sensitive" to security concerns decades after the end of the Cold War.

But the foreign minister described the nuclear explosions as "practical countermeasures" against the United States and a demonstration of the "strongest-ever will" of North Korea's ruling party and people.

Japan, South Korea raise alarm 

Japan and South Korea used their addresses at the General Assembly this week to raise alarm bells over the threat from North Korea.

Japanese Prime Minister Shinzo Abe demanded that the world find a new way to confront Pyongyang after a decade of UN sanctions failed to change its behavior.

"The threat to the international community has become increasingly grave and all the more realistic," he said on Wednesday. "It demands a new means of addressing it, altogether different from what we applied until yesterday."

 South Korea's Foreign Minister Yung Byung-se suggested that the North could be stripped of its status as a member of the United Nations for refusing to accept the Security Council's decisions.

"I believe it is high time to seriously reconsider whether North Korea is qualified as a peace-loving UN member," he said in his address on Thursday.

However, such a proposal is likely to be opposed by China, Pyongyang's ally, which has repeatedly called for a de-escalation of tensions.

North Korea's membership in the United Nations — the only major international forum where Pyongyang has a voice — dates back 35 years to when the two Koreas were admitted simultaneously.

China, which is in negotiations with the United States on a new sanctions resolution, is also pushing for a resumption of six-party talks on North Korea's nuclear programme.

Yung accused North Korea's government of engaging in a "fanatical and reckless pursuit" of nuclear and missile programmes that had reduced the country to a wasteland of poverty and oppression. 

Also voicing alarm, the North Korean foreign minister declared that the Korean peninsula had "turned into the world's hot spot" that could see "the outbreak of nuclear war".

 But blame for the "root cause" of the crisis lies with the United States, he added, railing again against US-South Korea military exercises held twice this year.

BURLINGTON, Washington — A manhunt was underway on Saturday in northwest Washington state for a gunman who entered a Burlington shopping mall with a rifle the previous evening and killed five people before disappearing under the cover of darkness, the state police force said.

A man walked into the Cascade Mall, around 105 km north of Seattle, and began shooting at about 7pm local time on Friday, Washington State Patrol spokesman Mark Francis said on Twitter. The attack took place in the cosmetics section of a Macy’s department store, Francis told The New York Times.

Four women were initially killed in the rampage, which state police believe was carried out by a lone gunman. Later a man who was taken to a local hospital with serious injuries died, Francis said in a statement early on Saturday morning.

Authorities offered no information about a possible motive for the attack, which followed a series of violent outbursts at shopping center across the United States, including the stabbing of nine people at a Minnesota center last weekend.

There was no indication from authorities that the rampage was linked to any previous attack, and none of the victims were identified.

Local and state police officials were not immediately available for further comment but were expected to hold a news conference later on Saturday morning.

Francis said on Twitter that police and rescue workers had carefully worked their way through the mall, clearing stores and evacuating shoppers, some of whom locked themselves in dressing rooms.

He also said police were searching the area for the shooter, who was described as a “Hispanic male wearing gray” and was last seen walking toward a nearby interstate highway. The mall is located 72.5km south of the border with the Canadian province of British Columbia.

State police released a grainy photo of the suspect taken by a surveillance camera. It showed a young man who appeared to be in his 20’s with short dark hair and a pale complexion and carrying a rifle.

Local authorities searched throughout the night for the gunman, believed to be armed with a rifle, and warned residents to remain inside. Burlington is 72.5km south of the border with the Canadian province of British Columbia.

“Tragedy has struck in Washington tonight. Our hearts are in Burlington,” Governor Jay Inslee said on Twitter.

The shooting comes less than a week after a man stabbed nine people at a mall in the central Minnesota city of St Cloud before being shot dead by an off-duty police officer. The FBI is investigating that attack as a potential act of terrorism.

“At this time, the FBI has no information to suggest additional attacks planned in WA state,” the Washington State Patrol said on Twitter.

CHARLOTTE, N.C. — Police in Charlotte do not plan for now to release a video showing the fatal shooting of a black man by officers that has sparked two nights of violent protests in North Carolina's largest city, the department's chief said on Thursday.

The video will only be shown to the family of Keith Scott, 43, who was shot dead by a black police officer in the parking lot of an apartment complex on Tuesday afternoon, Charlotte-Mecklenburg Police Chief Kerr Putney said.

North Carolina Governor Pat McCrory declared a state of emergency during Wednesday night's rioting, which saw one man critically wounded by a gunshot. At least eight more civilians and four police officer were injured and 44 people arrested for charges ranging from assault to failure to disperse.

Many of the protesters dispute the official account of Scott's death. Police contend he was carrying a gun when he approached officers and ignored repeated orders to drop it. His family and a witness say he was holding a book, not a firearm, when he was killed.

"I'm not going to release the video right now," Putney told reporters, the morning after nine people were injured and 44 arrested in riots over Scott's killing.

He said the video supports the police account of what happened, but does not definitively show Scott pointing a gun at officers.

Charlotte's reluctance to release the video stands in contrast to Oklahoma, where officials on Monday released footage of the fatal shooting of Terence Crutcher by police after his vehicle broke down on a highway. That shooting is now the subject of a US Department of Justice probe.

The killings were the latest in a long series of controversial fatal police shootings of black men across the United States, sparking more than two years of protests asserting racial bias and excessive force by police and giving rise to the Black Lives Matter movement.

Scott's killing was the 214th of a black person by US police this year out of an overall total of 821, according to Mapping Police Violence, an anti-police violence group created out of the protest movement. There is no national-level government data on police shootings.

Overnight, protesters smashed windows and glass doors at a downtown Hyatt Hotel and punched two employees, the hotel's manager told Reuters. The slogan "Black Lives Matter" was spray-painted on windows.

Looters were seen smashing windows and grabbing items from a convenience store as well as a shop that sells athletic wear for the National Basketball Association's Charlotte Hornets. Protesters also set fire to trash cans.

No curfew imposed

Hundreds of additional state police officers and National Guard troops would be deployed to Charlotte's streets on Thursday to prevent a repeat of the violence, Putney said. But officials said they had no plans to impose a curfew.

"It should be business as usual," Putney said. "We don't see the need to definitively shut the city down at a specific hour."

However, large Charlotte employers including Bank of America Corp. and Wells Fargo & Co. told employees not to go to uptown offices on Thursday but work from home.

Officials initially said that a man had died during the protests and also that he had been shot by a civilian. Putney on Thursday said the department was looking into allegations that he had been shot by a police officer.

"We're here to seek the truth, so we're investigating that to find the truth, the absolute truth as best as the evidence can show us," Putney said.

The American Civil Liberties Union has called on the police in Charlotte to release camera footage of the incident. Authorities have said the officer who shot Scott, Brentley Vinson, was in plainclothes and not wearing a body camera. But according to officials, video was recorded by other officers and by cameras mounted on patrol cars.

Todd Walther, the Charlotte Fraternal Order of Police official, said the plainclothes officers were wearing vests marked "police" and that he saw them do nothing wrong. Releasing the video would satisfy some people, but not everyone, he added.

"The clear facts will come out and the truth will come out. It's unfortunate to say that we have to be patient, but that's the way it's going to have to be," Walter said.

ISLAMABAD — Pakistan's air force closed a major highway on Thursday to let it practice landing jets on the road, in what it said was routine training not related to heightened tension with India after a deadly attack in the disputed Kashmir region.

The attack in the frontier town of Uri — the latest in a decades-long dispute over Kashmir, claimed and held in part by both India and Pakistan — has raised new fears of military conflict between the nuclear-armed neighbours.

India has accused Pakistan of being behind Sunday's attack on an army base that killed 18 soldiers in the Indian-controlled portion of Kashmir and said it had the right to respond.

Pakistan has rejected the allegation and accused India of apportioning blame before the incident had been investigated properly.

On Thursday, traffic on Pakistan's busy main highway between Islamabad, the capital, and the eastern city of Lahore was diverted to an older mountain road during the two-day air force exercise, dubbed High Mark.

"They landed on the road in this, yes. That is something they have been doing for years," Pakistan air force spokesman Commodore Javed Mohammad Ali said. The drill was needed "in case your runways get damaged or they are not available for you", he added.

The exercise was not ordered in response to recent tensions with India and the timing was a coincidence, he said.

"This exercise, High Mark, is not done overnight just like that," he said, describing it as "a routine training matter".

‘High alert’

Another security official, however, said the Pakistani military was on high alert in case India decided to retaliate for the Uri attack with cross-border military force.

So far, India's military response to the Uri attack has been limited to skirmishes near the Line of Control separating the countries in Kashmir.

The Indian army said on Thursday it had foiled two attempts by militants to infiltrate into Indian-controlled Kashmir on Wednesday night.

India has long accused Pakistan of playing a role in the 27-year-long insurgency against its rule in Jammu and Kashmir, its only Muslim-majority state.

Calls for a military response have increased since the Uri attack, particularly from some politicians within Prime Minister Narendra Modi's ruling nationalist Bharatiya Janata Party.

Pakistan denies sending fighters into Indian-administered Kashmir but voices support for separatists seeking to end Indian rule there. The countries have fought two of their three wars over the Himalayan region.

India's portion of Kashmir has been under a major security lockdown during more than two months of protests triggered by the July 8 killing by Indian security forces of Burhan Wani, a popular young commander of the Kashmiri separatist group Hizbul Mujahideen, whose leader is based in Pakistan.

In New York, Pakistani Prime Minister Nawaz Sharif called on the UN General Assembly to investigate atrocities Pakistan alleges that Indian forces have committed in Kashmir.

At least 78 civilians have been killed and thousands wounded in more than two months of clashes between protesters and Indian security forces there. 

OSAN, South Korea — Two US supersonic bombers flew over South Korea on Wednesday, with one of them landing at an air base 40km south of the capital, the second such flight since North Korea's September 9 nuclear test.

US Forces Korea said the flight by a pair of B-1B Lancer strategic bombers based in Guam was a show of force and of US commitment to preserve the security of the peninsula and the region.

The United States, which has about 28,500 troops in South Korea, flew two B-1 bombers on September 13 escorted by US and South Korean fighter jets in a show of solidarity with Seoul.

The North condemned the earlier flight as an armed provocation that mobilised "ill-famed nuclear killing tools". It did not immediately respond to Wednesday's flight.

The US Air Force said the Wednesday flight was the closest ever to North Korea by a B-1 bomber.

"Today marks the first time the airframe has landed on the Korean peninsula in 20 years, as well as conducting the closest flight near North Korea ever," the US Air Force said on its website which also showed a B-1B bomber landing at the base in South Korea.

The South's Yonhap news agency said the aircraft flew over a US live-fire training site in the Pocheon area bordering the North.

North Korea has ignored global condemnation of its fifth nuclear test on September 9, and this week said it had successfully tested a new rocket engine that would be used to launch satellites, again in violation of UN sanctions.

The leaders of the United States and China, which is the North's main diplomatic ally and economic benefactor, condemned the latest nuclear test and pledged to step up cooperation at the United Nations and in law enforcement channels.

China urges restraint

UN diplomats say the two countries have begun discussions on a possible UN resolution in response to the latest nuclear test, but China has not said directly whether it would support tougher steps against North Korea.

China, which has objected to a planned US deployment of a THAAD missile defence system in the South to counter the North's missile threat, called on "all parties to exercise restraint and to avoid any actions that could further escalate tensions".

South Korea's prime minister, Hwang Kyo-ahn, told parliament South Korea wanted existing UN sanctions against the North tightened by removing loopholes that allow it to trade in minerals if it is for subsistence.

North Korea has been testing nuclear weapons and ballistic missiles at an unprecedented rate this year, beginning with its fourth nuclear test in January and including the launch of a satellite in February that was widely seen as a test of long-range ballistic missile technology.

The North's test of a new rocket engine for satellite launchers this week was believed to be part of a long-range missile programme, according to the South's military.

North Korea's leader, Kim Jong-un, ordered preparations for the launch of a satellite "as soon as possible" on the basis of the successful test, its state media reported.

North Korea this month fired three missiles that flew about 1,000km, and in August tested a submarine-launched ballistic missile that experts said showed considerable progress.

It also launched an intermediate-range missile in June that experts said marked a technological advance for the isolated state after several failed tests.

South Korean Defence Minister Han Min-koo told parliament the North was developing all types of missiles, from short- to long-range, and its advances were "considerable".

CHARLOTTETOWN, Prince Edward Island — Canadian police evacuated all schools in the eastern province of Prince Edward Island on Wednesday after a bomb threat, but found no explosive device.

About 19,000 students at 62 schools were taken to safe locations in their communities around the island off Canada's Atlantic coast. It is the country's smallest province with a population of about 146,000.

"The minister of public safety just informed me that all of the schools' children and personnel are safe. The situation is under control," Prime Minister Justin Trudeau said at a news conference in Ottawa.

In Prince Edward Island's capital of Charlottetown, Royal Canadian Mounted Police Sergeant Kevin Bailey said at a press conference he was aware of separate threats in the nearby province of Nova Scotia and the United States.

He said later in a telephone call that police analysts had identified threats against schools in North Carolina, Connecticut and Arizona, but there was no immediate indication that they were linked.

Bailey said the threat against Prince Edward Island schools came by fax to the police in Ottawa.

"The message stated that the bombs had been placed in a number of schools and would be detonated today but it did not specify the time," Bailey said. "There is an active investigation to identify the person or persons responsible."

The University of Prince Edward Island said in note on its website that it was closed for the day, "given the situation happening across the province".

"They didn't say there was a bomb threat or any specific threat; they just told us that school had been suspended for the rest of the day," Charlottetown student Peter Keedwell, 17, said in a telephone interview. "It was pretty calm... People weren't running out of school or anything."

Parker Grimmer, director of Prince Edward Island's public schools branch, said students would be able to return to school on Thursday.

Police in Nova Scotia closed some college and university campuses. A Nova Scotia Community College campus in Halifax was evacuated after an anonymous threat, Halifax Regional Police said in a press release.

It said an explosive disposal unit had gone to the scene but found that a suspicious package was only garbage and reopened streets in the area.

The college's Marconi campus in Sydney, Nova Scotia, also shut down. Cape Breton University, also in Sydney, said on its website that it was closed for the day.

UNITED NATIONS — In a closing dispatch to the world he's tried to shape, President Barack Obama conceded Tuesday that the United States and other major powers have only limited ability to solve the world's most profound problems, including Syria's civil war. He lamented the "cycles of conflict and suffering" that seem to kick in every time humanity finally seems to be getting it right.

"Perhaps that's our fate," Obama said in his last speech to the UN General Assembly.

Four months before leaving office, Obama called for a "course correction" to ensure that the unstoppable forces of globalisation don't lead nations to entrench behind their borders and ignore the most vulnerable. He chided foreign leaders for stoking ethnic and religious divisions while faulting Russia for a brutish approach to its role on the world stage.

Still, Obama insisted it was critical not to gloss over "enormous progress" on economics and global cooperation that he said formed a template for tackling the problems of the future.

In a less-than-subtle jab at Donald Trump, the Republican running to replace him, Obama said, "A nation ringed by walls would only imprison itself."

Obama's parting words to the global body contained a grim assessment of the challenges he's leaving behind: a devastating refugee crisis, terrorism, financial inequality and a tendency to make immigrants and Muslims scapegoats. Across the Middle East, he said, "basic security, basic order has broken down".

"This is the paradox that defines our world today," Obama said. "A quarter-century after the end of the Cold War, the world is by many measures less violent and more prosperous than ever before. And yet our societies are filled with uncertainty and unease and strife."

This year's UN gathering has played out against the harrowing backdrop of the deepening civil war in Syria and the renewed failure of US and Russian diplomatic efforts to stem the violence for any meaningful period of time. With no better alternatives, the US, Russia and others clung unconvincingly on Tuesday to the notion that a week-old cease-fire was not moribund, even as Syria declared it over and the UN suspended all convoys of aid.

Obama acknowledged that the extremist and sectarian violence wreaking havoc in the Middle East and elsewhere "will not be quickly reversed". Still, he stuck faithfully to his insistence that diplomatic efforts and not military solutions are the key to resolving Syria's war and other conflicts.

"If we are honest, we know that no external power is going to be able to force different religious communities or ethnic communities to co-exist for long," Obama said. "Until basic questions are answered about how communities co-exist, the embers of extremism will continue to burn. Countless human beings will suffer."

The president was unabashed in his critique of Russia as he laid out his diagnosis of the world's ills. His tough talk illustrated how little progress has been made in reconciling the two powers' diverging interests that have allowed the Syria crisis to continue to fester.

"In a world that left the age of empire behind, we see Russia attempting to recover lost glory through force," Obama said.

A year ago, Obama stood at the same podium and declared anew that Syrian President Bashar Assad must leave power, while Russian President Vladimir Putin gave a dueling speech warning it would be a mistake to abandon Assad. Since then, Moscow's leverage in the conflict has strengthened significantly, buoyed by a Russian military intervention that bolstered Assad's standing without pulling it into the military "quagmire" that Obama had predicted.

White House officials said Obama had been mindful of the fact that his UN speech was one of his final opportunities to define his leadership on the world stage. At the heart of his approach, Obama said, is a belief that conflicts are best solved when nations cooperate and a willingness to engage with erstwhile adversaries like Cuba and Myanmar.

It's a theme that Democrat Hillary Clinton has put at the forefront of her presidential campaign — her slogan is "Stronger Together" — as she casts herself as the natural heir to Obama's legacy. The president peppered his speech with subtle references to Trump, his calls for building a wall on the Mexican border and his denigration of Muslims and immigrants.

Standing before the 193-member UN body, Obama sought in broad strokes to lay out a blueprint for addressing other unresolved conflicts. He called for the world to impose "consequences" on North Korea for its latest nuclear test and, in less direct terms, for China to abide by a recent UN tribunal ruling against its territorial designs in the South China Sea.

A day before meeting with Israeli Prime Minister Benjamin Netanyahu, he drew a parallel between the Israeli-Palestinian conflict and the need to respect racial minorities in the US.

"Surely, Israelis and Palestinians will be better off if Palestinians reject incitement and recognise the legitimacy of Israel, but Israel recognises that it cannot permanently occupy and settle Palestinian land," Obama said.

NEW YORK — Donald Trump's hardline stance on immigration after two bombings in the New York area again sparked debate on the White House campaign trail on Tuesday, as his son sparked a firestorm by comparing Syrian refugees to lethal candies.

The Republican presidential hopeful's uncompromising position on illegal migrants — even calling them a dangerous "Trojan horse" who enter the country with the aim of doing harm — is inextricably linked with his meteoric political rise.

Most Republicans approve of his tough talk, with polls showing that a majority of party members agreed with his call last December to bar Muslims from entering the United States. 

Since then, Trump has refrained from specifically targeting Muslims, but he has championed police profiling of suspects and promised to bar immigrants and travellers from certain countries deemed dangerous, like Syria.

The candidate has repeatedly warned of the risks posed by Syrian refugees, citing the arrival in Europe of Daesh operatives disguised as simple refugees, and says US immigration officials are not properly screening new arrivals.

But immigration is nevertheless not one of the major issues on the minds of voters, placing only fourth according to a recent CBS/New York Times poll, making it unclear if Trump, 70, can use it to persuade those who are still undecided.

But national security and the anti-terror fight are number two on that list, and with less than 50 days to go before Election Day on November 8, Trump is attempting to blur the lines between the two issues.

"This is a question of quality of life," Trump told a rally in key swing state Florida on Monday. "We want to make sure we are only admitting people in our country who love our country."

Trump's son Donald Jr, one of his surrogates on the campaign trail, has meanwhile courted controversy with a tweet comparing Syrian refugees to a bowl of multi-colored Skittles candy — some of them lethal, but not to the naked eye.

"This image says it all," he said on Twitter, with a picture of a white bowl filled with the popular sweets.

Written above the image is: "If I had a bowl of skittles and I told you just three would kill you. Would you take a handful? That's our Syria refugee problem." 

Trump Jr thus argued that it is too dangerous to welcome migrants when a tiny number of them could later launch attacks. The man accused of planting bombs in both downtown Manhattan and near a race in New Jersey is a naturalised American of Afghan descent.

At his rallies, Trump Sr has repeatedly recited the lyrics to a 1960s song, "The Snake", which recounts how a woman who aided an injured snake ended up being bitten, and used the Trojan Horse imagery to warn of hidden dangers.

"We cannot let this evil continue. Cannot do it," he said on Monday.

"Nor can we let the hateful ideology of radical Islam, its oppression of women, gays, children and non-believers, be allowed to reside or spread within our country."

Wrigley, the maker of Skittles, on Tuesday criticised Trump Jr's tweet.

"Skittles are candy. Refugees are people. We don't feel it's an appropriate analogy," a company statement said.

"We will respectfully refrain from further commentary as anything we say could be misinterpreted as marketing."

Trump's Democratic rival Hillary Clinton, who had no public events scheduled on Tuesday a week after taking a break from campaigning to recover from pneumonia, backs President Barack Obama's position on immigration.

Obama has increased the number of Syrian refugees granted entry to the United States in the face of the brutal five-year war in their homeland.

Her campaign team published the story of an American of Libyan descent, who helps refugees to settle in the US.

"Donald Trump would have kicked my family out of the country," said the man from swing state Ohio, identified as Mohamed G.